The objective of the proposed research is to initiate a behavior-genetic analysis of the associative behavior, Pavlovian counterconditioning, in the blow fly Phormia regina. Counterconditioning, especially counterconditioning that involves temporal and continent pairing of unconditioned stimuli, is believed to depend on the interaction between two internal states, the central appetitive and the central aversive states. In the blow fly these two states probably correspond to the Central Excitatory State (CES) and Central Inhibitory State (CIS). The CES has been extensively studied in this and other laboratories, and it has been shown to be involved in the expression of a second form of learning, Pavlovian excitatory conditioning. The research outlined in this proposal will examine the relationship between counterconditioning and the CES and the CIS. Specifically, this will involve (a) determining the reliability and stability of individual differences in counterconditioning, (b) investigating the relationship between CES and counterconditioning in individual flies, (c) confirming the biological significance of any observed correlations by hybridization analysis, and (d) developing a reliable and stable measure of individual differences in CIS. To carry out these investigations I will use lines of flies that are fixed for the alleles that are correlated with high and low CES expression. Such genetically-fixed lines allow me to "preset" or manipulate the expression of CES independently of the counterconditioning or CIS paradigms. Ultimately, similar CIS lines will be obtained by selection and/or single-pair matings. Since counterconditioning depends on the intensity of the CIS and CES, and the phenotypes of counterconditioning depends on the intensity of the CIS and CES, and the phenotypes of these states reflect neural, physiological and genetic events, behavior-genetic investigation of counterconditioning should lead to a better understanding of the biological mechanisms underlying learning.